Color improvement of phosphate esters

ABSTRACT

A method for decolorizing and stabilizing alkylphenyl esters of phosphoric acid which comprises contacting the esters with an effective amount of a triorganoaluminum or organoaluminum hydride for a sufficient length of time to decolorize and stabilize the esters against subsequent color formation.

BACKGROUND OF THE INVENTION

This invention relates to the decolorization and stabilization of alkylphenyl esters of phosphoric acid. The production of low color and color stable phosphate esters from alkylphenol precursors has been a long recognized problem. In the conventional production of phosphate esters from alkylphenols, undesirable color formation frequently occurs. Color can decrease the phosphate ester's value as a commercial product.

Alkylphenyl esters of phosphoric acid find extensive use as plasticizers for nitrocellulose and polyvinyl chloride (PVC) compositions. In addition, they also serve as additives for gasoline, functional fluids, oils, and are useful as flame retardants in plastics, and the like.

The preparation of alkylphenyl esters of phosphoric acid is generally accomplished by the addition of phosphorus oxychloride, (POCl₃) to selected phenols, such as cresols, xylenols, and the like, and gradually heating the resulting reaction mixture to about 180°C. The reaction is accelerated by the presence of a Friedel-Crafts catalyst such as aluminum chloride, (AlCl₃). In conventional processing, the reaction product is vacuum distilled to remove unreacted phenols as an initial fraction, and the alkylphenyl phosphate ester as a product fraction, leaving high boiling point materials and the catalyst in the residue. The distilled product fraction is then washed thoroughly with sodium hydroxide solution to remove free phenol and acidic materials, followed by water washing. The product fraction is then generally treated with activated carbon to remove color causing impurities.

This process, which works very well with esters produced from conventional by-product alkylphenols, produces unsatisfactory material when applied to mixed alkylphenols produced by the alkylation of phenol with olefins. It is found that the products discolor upon exposure to air, exposure to heat, or storage in the dark. The discoloration has been attributed to the presence of di(o-alkyl) phenols in the alkylated phenol. Steric hindrance caused by the double ortho substitution in 2,6-dialkylphenols and in 2,4,6-trialkylphenols renders the phenols unresponsive to washing with sodium hydroxide solution, so that they are not removed by the caustic wash.

These so-called "hindered phenols," can oxidize in the presence of air to form highly colored quinones, which are the source of undesirable discoloration in the product. These quinones can bleach somewhat in the light, however, color will reappear upon storage in the dark. The color can intensify when the ester is mixed or milled with polyvinyl chloride (PVC) under the influence of air and heat.

Thus, for example, in the case of 2,6-diisopropylphenol, the corresponding diphenoquinone or benzoquinone is highly colored. Esters made from mixtures of alkylated phenols which contain di-orthoalkylphenols such as 2,6-diisopropylphenol can be too highly colored for many uses, particularly for use as plasticizers. High color phosphate esters have limited utility for plasticizer use and are less marketable.

Color formation in phosphate esters is discussed in U.S. Pat. No. 3,681,482, which correlates the degree of color formation in phosphate esters to the degree of alkyl substitution of the aryl ring. Accordingly, tris(methylphenyl) phosphate will produce less color than tris(dimethylphenyl) phosphate. This may be explained by the fact that the tris(methylphenyl) phosphate has only one methyl group substituted on the aryl ring, whereas tris(dimethylphenyl) phosphate has two methyl groups substituted on the aryl ring and, therefore, has more color.

A number of proposals have been made for methods to overcome the problem of undesirable color formation. For example, U.S. Pat. No. 1,958,210 discloses the use of activated carbon to decolorize and remove oxidizable impurities from phosphate esters. This approach is unsatisfactory because activated carbon is not an effective decolorizing agent for alkylphenyl phosphate esters. In certain instances, for example, in the decolorization of isopropylphenyl diphenyl phosphate ester, the use of activated carbon may increase color formation.

U.S. Pat. No. 2,113,951 discloses a method wherein an alkylphenol such as cresylic acid is distilled in the presence of a mineral acid such as sulfuric, hydrochloric or phosphoric acid, to purify it. The purified cresylic acid is then employed in the manufacture of tricresyl phosphate esters which are supposed to be more stable to the action of heat and light than the corresponding ester made from alkylphenols distilled in the absence of an inorganic acid. The disadvantage of this process is that the phenolic residues oxidize to colored quinones, and must be thoroughly distilled in order to remove them and avoid further color formation.

Another method for reducing color is proposed in U.S. Pat. No. 3,681,482 wherein sodium borohydride is used to permanently bleach and color stabilize tris(alkylphenyl) phosphate esters containing 2,6-diisopropylphenol and the corresponding diphenoquinone. The sodium borohydride reduces the diphenoquinone to the colorless 2,6-diisopropylphenol which, however, remains in the product and is a potential source of discoloration if the product is exposed to oxidizing conditions. Sodium borohydride treatment is also expensive in cost of materials and time, as several hours to overnight treating times are necessary.

Thus, it can be seen that the methods proposed in the prior art, are not commercially effective for removing color from alkylphenol esters of phosphoric acid, or do not improve the PVC mill stability when these esters are used as PVC plasticizers.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method for removing color from alkylphenyl esters of phosphoric acid which also improves the mill stability of these esters when they are used as plasticizers for PVC has now been discovered.

The method comprises contacting alkylphenyl phosphate esters with an effective amount of a triorganoaluminum or organoaluminum hydride, neat or preferably in a suitable mineral oil or hydrocarbon solvent for a time sufficient to reduce the color to the desired level. The phosphate ester is decolorized and stabilized against subsequent color formation. This treatment produces a low color, stable phosphate ester. The triorganoaluminum or organoaluminum hydride may be added neat or preferably in solution.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

Permanent removal of color from colored phosphate esters according to the method of this invention is achieved by contacting the phosphate ester with an effective amount of a triorganoaluminum or organoaluminum hydride, added neat, or preferably in solution for a time sufficient to decolorize the phosphate ester to the desired level, and stabilize it against subsequent color formation. Suitable solvents are pentane, hexane, heptane, octane, decane, isooctane, dodecane, xylene, tetralin, cyclododecane, mineral oils, octylbenzene, white oils, kerosenes, and the like.

This invention is applicable to all phosphate esters which are made from alkylated phenol mixtures which contain hindered phenols, e.g., phenols containing alkyl groups on both positions ortho to the hydroxyl group. The esters may contain 0.5 to 3 alkylarly groups and 0 to 2.5 phenyl groups. Preferably, the triaryl phosphate esters treated by the process of this invention are a mixture of esters containing 1 to 2 alkaryl groups. The esters correspond to the general formula: ##EQU1## where R is alkylaryl and R₁ and R₂ may be alkyl, alkaryl, aralkyl or aryl, and wherein the alkyl groups can contain from 1 to 20 carbon atoms, more preferably from 1 to 12 carbon atoms. Some triphenylphosphate may also be present.

The alkylated phenols which contain hindered phenols are usually made by alkylating phenol with C₂ -C₁₂ unsaturated hydrocarbons such as ethylene, propylene, isobutylene and its isomers, amylene and its isomers, tripropylene, tetrapropylene, decene, dodecene, diisobutylene and the like.

Typical examples of alkyl radicals are as follows: methyl, ethyl, normal propyl, isopropyl, normal butyl, isobutyl, secondary butyl, tertiary butyl, normal amyl, isoamyl, 2-methylbutyl, 2,2-dimethylpropyl, 1-methyl butyl, diethylmethyl, 1,2-dimethyl propyl, tertiary amyl, normal hexyl, 1-methylamyl, 1-ethyl butyl, 1,2,2-trimethyl propyl, 3,3-dimethyl butyl, 1,1,2-trimethyl propyl, 2-methyl amyl, 1,1-dimethyl butyl, 1-ethyl 2-methyl propyl, 1,3-dimethyl butyl, isohexyl, 3-methylamyl, 1,2-dimethyl butyl, 1-methyl 1-ethyl propyl, 2-ethyl, normal heptyl, 1,1,2,3-tetramethyl propyl, 1,2-dimethyl 1-ethyl propyl, 1,1,2-trimethyl butyl, 1-isopropyl 2-methyl propyl, 1-methyl 2-ethyl butyl, 1,1-diethyl propyl, 2-methyl hexyl, 1,1-dimethyl amyl, 1-isopropyl butyl, 1-ethyl 3-methyl butyl, 1,4-dimethyl amyl, isoheptyl, 1-methyl 1-ethyl butyl, 1-ethyl 2-methyl butyl, 1-methyl hexyl, 1-1-propyl butyl, normal octyl, 1-methyl heptyl, 1,1-diethyl 2-methyl propyl, 1,1,3,3-tetramethyl butyl, 1,1-diethyl butyl, 1,1-dimethyl hexyl, 1-methyl 1-ethyl amyl, 1-methyl 1-propyl butyl, 2-ethyl hexyl, 6-methyl heptyl normal nonyl, 1-methyl octyl, 1-ethyl heptyl, 1,1-dimethyl heptyl, 1-ethyl 1-propyl butyl, 1,1-diethyl 3-methyl butyl, diisobutyl methyl 3,5,5-trimethyl hexyl, 3,5-dimethyl heptyl, normal decyl, 1-propyl heptyl, 1,1-diethyl hexyl, 1,1-dipropyl butyl, 2-isopropyl 5-methyl hexyl and C₁₁ -C₂₀ alkyl groups.

Also included are aralkyl groups, e.g., benzyl, alpha- or beta-phenylethyl, alpha,alpha dimethylbenzyl and the like. Also included are cyclohexyl, cycloheptyl, cyclododecyl, and the like.

Typical examples of aryl and alkaryl radicals are phenyl, cresyl, xylyl, alkoxylated phenyl, isopropylphenyl, butylphenyl, alpha-alkylbenzylphenyl and alpha, alpha-dialkylbenzylphenyl, e.g., alpha-methylbenzylphenyl, alpha, alpha dimethylbenzylphenyl, tert-nonylphenyl amylphenyl, tert-butylphenyl, isooctylphenyl, dodecylphenyl, tertiary octylphenyl and the like.

The invention is hereinafter exemplified by first showing the preparation of an ester via the alkylation of phenol with an olefin, followed by addition of POCl₃. These alkylated phenols produce esters which are similar to those produced with conventional by-product coal tar cresylic acids of methylphenols.

The esters are generally made by reacting an alkylphenol with POCl₃ in the presence of a Friedel-Crafts catalyst at an elevated temperature, typically about 180°C., until the reaction is complete, as noted by the cessation of HCl evolution. The reaction mixture is then heated to distill excess phenols overhead. The temperature and/or vacuum is then increased and the phosphate ester product is distilled leaving the catalyst and a small amount of high boiling distillation residue.

Conventionally, the ester product is washed with aqueous alkali to remove free phenols which are generally present in the range of about a few tenths of a percent. The washed product is separated from the water and generally treated with activated carbon and a filter aid, such as diatomaceous earth, and filtered. However, product discoloration caused by hindered phenols in the presence of air and heat can ensue, rendering the product unsuitable for use in applications where lack of color is important.

In accordance with this invention, the phosphate ester is treated for color removal and color stabilization by contacting the alkylphenyl phosphate ester with an effective amount of a triorganoaluminum or organoaluminum hydride for a sufficient length of time until the color is adequately reduced. The contacting treatment is preferably conducted during the final treatment of the finished product, at a temperature of about 10°C. to about 100°C. and preferably at about 20°C. to 70°C. for a period of about 5 to about 300 minutes, and preferably about 15 to about 90 minutes. The solvent can be removed in a conventional manner, such as by distillation or vacuum stripping.

The triorganoaluminum and organoaluminum hydrides used in this invention have the general formula:

    R'.sub.n AlH.sub.m

wherein R' is a radical selected from the group consisting of alkyl, cycloalkyl, aryl, alkaryl, or aralkyl and contains from 1 to 20 carbon atoms, preferably 1-12 carbon atoms and n varies from 1 to 3, and m varies from 0 to 2. Typical examples of the above radicals are as have been hereinbefore exemplified.

Treatment times will vary, generally from about 5 minutes to about 24 hours, depending upon the amount of phosphate ester treated, the amount and concentration of aluminum compound in solution, the temperature, agitation, and the like. Preferably, the phosphate esters are washed for about 1 hour to about 5 hours at a temperature of about 20°C. to about 100°C. More preferably, the washing treatment is carried out at temperatures of about 45°C. to about 70°C.

The amount of triorganoaluminum or organoaluminum hydride may vary in amount from about 0.01% to about 10% by weight of the phosphate ester treated. Larger amounts of the triorganoaluminum or organoaluminum hydride can be employed, but no advantage is accrued thereby. It is preferred to use an amount ranging from about 0.05% to about 5% by weight of the phosphate ester. The particular amount of organoaluminum compound employed in any given instance will to some extent be influenced by a number of factors, which include the amount of color present, the extent of color improvement desired, the particular phosphate ester treated, treatment time, and the like.

The method of this invention is generally conducted under atmospheric pressure. However, higher or lower pressures may be used. It may also be conducted under an inert atmosphere, such as nitrogen which serves to repress re-oxidation.

The triorganoaluminum and organoaluminum hydrides of the present invention may be added neat, or preferably in solution. The method of this invention may be carried out batch-wise or in a continuous manner.

One particular advantage of the instant invention is that after treatment of the phosphate ester with the organoaluminum compound, no additional steps or special treatment other than a conventional stripping or distillation step to remove the solvent, is necessary. Trace residual organoaluminum compounds if present, do not appear to have an adverse effect on the commercial properties of the phosphate esters.

The following examples are illustrative of the methods disclosed above, and are provided without any intention that the invention be limited thereto. In the examples and throughout the specification, all parts and percentages are by weight, unless otherwise noted.

EXAMPLE 1 Preparation of Isopropylphenyl Diphenyl Phosphate Ester

1.86 Moles of phenol, 1.50 moles of an isopropylphenol mixture containing mono- and diisopropyl phenols, and 1.12 moles of phosphorus oxychloride were placed in a 1 liter reactor and stirred at a moderate rate at room temperature. Nitrogen gas was bubbled slowly through the liquid and anhydrous magnesium chloride catalyst was added to the stirred contents. The charge was then heated, while slowly being stirred, from a temperature of 38°C. to 180°C. over a period of 4 hours. The temperature was maintained at 180°C. for approximately 5 hours or until the evolution of hydrogen chloride ceased.

The crude isopropylphenyl diphenyl phosphate product weighing 423 grams was then transferred to a 1/2 liter distillation flask and vacuum distilled through a 6 × 1 inches column packed with 1/4 inch glass spheres. The distilled isopropylphenyl diphenyl phosphate ester product weighing 366 grams was then introduced into a 2 liter reactor and washed for one hour at 65°C. with 300 milliliters of a 1% sodium hydroxide solution. The caustic wash was repeated two additional times with fresh caustic solution.

The isopropylphenyl diphenyl phosphate ester was then separated from the caustic solution and washed twice with 300 milliliters of water at 65°C. for 1 hour. The water phase after the second wash was neutral to pH paper. The washed ester is separated from the final wash water and dried at 100°C. under a vacuum of 1.0 millimeters Hg (5-10 millimeters Hg at 100°C. is also satisfactory), until no more water distills over. The dried ester is filtered to give 351 grams of finished ester product.

EXAMPLE 2

A 200 gram sample of a commercial finished isopropylphenyl diphenyl phosphate ester having a Saybolt Universal Viscosity of 150 seconds and prepared in a manner similar to that of Example 1 and having an APHA color value of 400 (American Public Health Association, Platinum-cobalt Scale for Designating Color) was contacted with 1 milliliter of a 25% solution of diisobutyl aluminum hydride in hexane. The solution was stirred at room temperature, and the APHA color value measured from various samples in accordance with the time as indicated in the Table below.

                  TABLE                                                            ______________________________________                                         Time in Minutes    APHA Color Value                                            ______________________________________                                         0                    400                                                       60                    75 to 100                                                110                   50 to 75                                                 ______________________________________                                    

The solution was then stripped of solvent under vacuum at 80°C/1.0 millimeters of Hg. The final APHA color value measured 50. The phosphate ester displayed excellent color stability when tested for PVC mill stability (which determines the effect of mill conditions on the darkening properties of a vinyl formulation incorporating the phosphate ester as a plasticizer). The untreated phosphate ester failed this PVC mill stability test. Storage tests were also performed over a two month period wherein the treated phosphate ester was placed in a dark area for two months with no change in color. It was also stable to a current of air flowing through it for 1 hour at 100°C.

EXAMPLE 3

A 200 gram sample of a commercial finished isopropylphenyl phenyl phosphate ester having a Saybolt Universal Viscosity of 220 seconds, prepared in the manner similar to that of Example 1, and having an APHA color value of 150 was contacted with 2 milliliters of a 25% diisobutyl aluminum hydride solution in hexane. The solution was stirred at 25°C. for 2 hours during which time the color decreased to an APHA value of 25 to 50. The solution was washed with 200 milliliters of a 0.5% HCl solution and two times with 200 milliliters of water. Upon phase separation, the product layer containing the phosphate ester was vacuum dried at 75°C./5 millimeters Hg to give a final APHA color value of 25 to 50. The treated ester passed the PVC mill stability test; untreated ester failed the test.

EXAMPLE 4

A 200 gram sample of a commercial finished isopropylphenyl diphenyl phosphate ester having a Saybolt Viscosity of 150 seconds prepared in a manner similar to that of Example 1 and having a APHA color value of 400 was treated by contacting with one milliliter of a 25% solution of triisobutyl aluminum in PRIMOL-355, a highly refined white mineral oil made by Exxon under nitrogen. After 4 hours of mixing at 25°C., the treated ester had and APHA color value of 75. The finished ester displayed an unusually marked color stability in the PVC mill stability test.

EXAMPLE 5

Similar results were obtained when a triethyl aluminum solution was substituted for the triisobutyl aluminum solution in Example 4. 

What is claimed is:
 1. In a method for producing alkylphenyl esters of phosphoric acid which comprises the alkylation of phenol with an olefin followed by the addition of POCl₃ in the presence of a Friedel-Crafts catalyst at an elevated temperature, the improvement which comprises contacting said esters with an effective amount of an organoaluminum compound in accordance with the formula:

    R'.sub.n AlH.sub.m

wherein R' is a radical selected from the group consisting of alkyl, cycloalkyl, aryl, alkaryl or aralkyl and contains up to 20 carbon atoms; and n varies from 1-3, and m varies from 0-2; for a period of time sufficient to reduce the color to the desired level whereby said esters are decolorized and stabilized against subsequent color formation.
 2. The method of claim 1 wherein said alkylphenyl esters correspond to the formula: ##EQU2## wherein R is alkaryl and R₁ and R₂ are selected from the group consisting of alkyl, aralkyl, alkaryl and aryl, and wherein the alkyl groups contain from 1 to about 20 carbon atoms.
 3. The method of claim 2 wherein the alkyl groups contain from 1 to about 12 carbon atoms.
 4. The method of claim 1 wherein the alkylphenyl esters contain unreacted phenols.
 5. The method of claim 1 wherein said organoaluminum compound is selected from the group consisting of triorganoaluminums, organoaluminum hydrides, and mixtures thereof.
 6. The method of claim 5 wherein said organoaluminum consists of a triorganoaluminum.
 7. The method of claim 6 wherein said triorganoaluminum compound consists of triisobutyl aluminum or triethyl aluminum.
 8. The method of claim 5 wherein said organoaluminum consists of an organoaluminum hydride.
 9. The method of claim 8 wherein said organoaluminum hydride consists of diisobutyl aluminum hydride.
 10. The method of claim 1 wherein said decolorizing and stabilizing is conducted under an inert atmosphere.
 11. The method of claim 5 wherein said inert atmosphere comprises nitrogen.
 12. The method of claim 1 wherein said organoaluminum compound is employed in solution.
 13. The method of claim 1 wherein the alkylphenyl ester of phosphoric acid is an isopropylphenyl phenyl phosphate.
 14. The method of claim 12 wherein solvent is removed after the treatment of completed.
 15. The method of claim 1 wherein residual aluminum compounds after treatment are removed by washing with dilute acid. 